Alright, so which definition of "On Sale" do you want to use/is relevant? Are we talking "Discounted" or "For Sale"? Because it's clearly for sale right now on Steam but you are right, it is not discounted.

When it will be discounted is basically unknown so if that's what the OP was asking about then I don't have an answer for him.

Due to the amount of time that Sims "Mideavil" has been available, I think we must assume that the OP was asking if anyone knew when the game would start to sell at a discount

That is probably a safe bet.

@OP, I have not checked my crystal ball in a while and the greek prophetess using the fumes in my basement recently died of carbon monoxide poisoning so I am unable to give you a satisfactory answer to your question.

At best you should check back during the Summer Sale in a few months time. It probably won't go on sale given it's pretty low profile status but they might have a Sims Bundle available so it might.